Trap (plumbing)
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In plumbing, a trap is a U-shaped portion of pipe designed to trap liquid or gas to prevent unwanted flow; most notably sewer gases from entering buildings while allowing waste materials to pass through. In oil refineries, traps are used to prevent hydrocarbons and other dangerous gases and chemical fumes from escaping through drains. In heating systems, the same feature is used to prevent thermo-siphoning which would allow heat to escape to locations where it is not wanted. Similarly, some pressure gauges are connected to systems using U bends to maintain a local gas while the system uses liquid. For decorative effect, they can be disguised as complete loops of pipe, creating more than one U for added efficacy.
Water seal in drain pipe under a sink. Water enters at right, fills the trap, and continues left. Inverted siphoning occurs below the line "A".
A bottle trap, which can be unscrewed for cleaning
Wash-out closet, manufactured by Myers Sanitary Depot, New York, 1890
Trap with copper drain pipe at underside of firestop packing in two-hour fire-resistance rated concrete floor slab
Sewer gas
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Sewer gas is a complex, generally obnoxious smelling mixture of toxic and nontoxic gases produced and collected in sewage systems by the decomposition of organic household or industrial wastes, typical components of sewage.
An old sewer gas chimney in Stonehouse, Plymouth, England, built in the 1880s to disperse sewer gas above residents
The cover of an 1882 issue of The Wasp, with an illustration linking sewer gas and disease